Voltage-controlled switching elements, such as IGBTs (Insulated-Gate Bipolar Transistors) and MOS transistors, have control terminals; selectively storing and dissipating electrical charge on and from the control terminal of a voltage-controlled switching element allows the voltage-controlled switching element to be selectively turned on and off. These voltage-controlled switching elements are used as H bridges for motor control. An H bridge is comprised of four switches designed to enable a load current to flow through a load in either direction.
Such an H bridge is illustrated in FIG. 5 of Japanese Examined Patent Publication No. 3339311; as each switch of the H bridge, a pair of an IGBT and a flywheel diode are used. The Japanese Examined Patent Publication No. 3339311 will be referred to as a known publication. As illustrated in the left view of FIG. 6 of the known publication, let us consider a case where a load current IL flows through a load 23 and a low-side IGBT 25 in the direction of arrow IL. In this case, turn-off of the IGBT 25 causes the load current IL to flow to a flywheel diode 28 for a corresponding higher-side IGBT 24 (see FIG. 5 of the known publication) and flow therethrough. This results in a reduction of a collector current IC, which has been flowing through the IGBT 25. Stray inductance LS in a line connected to both ends (emitter and collector) of the conductive path of the IGBT 25 and the rate of reduction in the collector current IC, which will be referred to as −dIC/dt, produce a surge ΔVp. The surge ΔVp is applied to the IGBT 25 and a flywheel diode 29 (see FIG. 5 of the known publication). The surge ΔVp is illustrated in the lower side of the right view of FIG. 6.
In order to reduce the surge ΔVp, it is necessary to reduce the rate (−dIC/dt) of reduction in the collector current IC, and in order to reduce the rate (−dIC/dt) of reduction in the collector current IC, it is necessary to reduce the switching speed of the IGBT 25.
Thus, the known publication discloses a driver for a voltage-controlled switching element, such as an IGBT, which is capable of reducing the switching speed of the voltage-controlled switching element upon detection of a decrease in a current, such as a collector current, flowing through the voltage-controlled switching element.
Specifically, this approach provides a driver for a voltage-controlled switching element, such as an IGBT; the driver includes two discharge paths connected to the voltage-controlled switching element and having different resistance values. The driver also includes a path to which a current measurement signal indicative of the magnitude of a current flowing through the voltage-controlled switching element is inputted. The driver is operative to selectively store electrical charge on the control terminal of the voltage-controlled switching element and dissipate the electrical charge stored in the control terminal of the voltage-controlled switching element therefrom via the two discharge paths. This approach is designed such that the driver opens one of the two discharge paths having the higher resistance value while closing the other having the lower resistance value when the current measurement signal indicates that a current flowing through the voltage-controlled switching element drops. This approach makes it possible to reduce the rate of reduction of a current flowing through the voltage-controlled switching element, thus reducing a surge produced based on the rate of reduction of the current flowing through the voltage-controlled switching element.